Critical phonons and R- zero-phonon emission line of Cr3+ in Mg-doped SrTiO3

Dielectric permittivity epsilon and a "dielectric related" temperature shift of a zero-phonon emission R line (E-2 --> (4)A(2)) of the octahedral Cr3+ impurity centers was studied for nominally pure SrTiO3 (STO), Sr0.99Mg0.01TiO3 (SMT) and SrTi0.99Mg0.01O3 (STM) ceramics. In STO, as in crystals, epsilon increases at cooling revealing only a weak dispersion in the 100 Hz(-1) MHz region at lower temperatures and the R line shifts at cooling to lower energies, peaking at approximate to 12605 cm(-1) at 25 K. In SMT, the permittivity reveals qualitatively the same behavior as STO. But the spectral position of the chromium R line appeared to be at lower energies (12575 cm(-1), 70 K) and furthermore the R- line shifts to lower energies at heating. In STM the permittivity reveals a wide maximum at similar to36 K with pronounced relaxation-type dispersion, hinting a possibility of a low-temperature phase transition (PT). The spectral position of the R line in STM is approximate to 12614 cm(-1) at 70 K (close to that one in STO). As in STO, at cooling the R line shifts to lower energies, but at similar to28 K the lineshift changes in sign. Such behavior could be associated with a low-temperature soft mode driven PT in STM. However, time resolved THz spectroscopy revealed no epsilon (T) anomaly in STM, which means that at least below 28 K the temperature dependence of the R line shift is not controlled by the TO1 soft mode. (C) 2003 Elsevier Science B.V. All rights reserved.